In recent years, research and development have been extensively conducted on light-emitting elements utilizing electroluminescence (EL). In a basic structure of such a light-emitting element, a substance with a light-emitting property is interposed between a pair of electrodes. By application of voltage between the electrodes, light emission can be obtained from the substance with a light-emitting property.
Since such a light-emitting element is of a self-light-emitting type, the light-emitting element has advantages over a liquid crystal display in that the viewing angle is wide and the visibility is high, and moreover the response speed is high and reduction in thickness and weight is possible.
Light-emitting elements utilizing electroluminescence are classified according to whether a substance with a light-emitting property is an organic compound or an inorganic compound. In general, the former is referred to as an organic light-emitting element, and the latter as an inorganic light-emitting element. These light-emitting elements are different from each other not only in their substances with a light-emitting property but also in their light-emitting mechanisms and features.
Among these light-emitting elements, an inorganic light-emitting element 1500 having a double insulating structure is known in which a light-emitting layer 1506 is sandwiched by insulating layers (a first insulating layer 1504 and a second insulating layer 1508) between a pair of electrodes (a first electrode 1502 and a second electrode 1510) as shown in FIG. 15. Such an inorganic light-emitting element provides light emission by application of AC voltage between the pair of electrodes (for example, see Reference 1: Japanese Published Patent Application No. H6-96861).
Inorganic light-emitting elements are classified according to their element structures into a dispersed inorganic light-emitting element and a thin-film inorganic light-emitting element. They are different from each other in that the former includes a light-emitting layer in which particles of a light-emitting material are dispersed in a binder and the latter includes a light-emitting layer formed of a thin film of a light-emitting material. However, they are common in that they require electrons accelerated by a high electric field.
Note that light emission mechanisms of the inorganic light-emitting elements include localized light emission which utilizes inner-shell electron transition of metal ions and donor-acceptor recombination light emission which utilizes a donor level and an acceptor level. In general, it is often the case that thin-film inorganic light-emitting elements employ localized light emission, and dispersed inorganic light-emitting elements employ donor-acceptor recombination light emission.
Since the inorganic light-emitting elements have a light emission mechanism by which light emission is obtained by collisional excitation of an electron accelerated by a high electric field against light emission center, a voltage of several hundred volts needs to be applied to the inorganic light-emitting elements. This causes a problem of increase in drive voltage in a case of applying the inorganic light-emitting element to a display panel and the like.
In order to solve the aforementioned problem, various element structures have been examined. For example, Reference 2 (Japanese Published Patent Application No. 2004-207246) suggests an element structure which aims at capturing a large amount of charges on a surface of a dielectric layer and decreasing drive voltage by providing an electric field enhancing layer made of carbon nanotube between a dielectric layer and an electrode.